Inject a small amount in vein, possibly this?
I believe the answer is:
vaccine
'vein possibly this?' is the definition.
The definition suggests a singular noun which matches the answer.
'inject a small amount in' is the wordplay.
'inject' becomes 'vine' (I can't explain this - if you can you should believe this answer much more).
'small amount' becomes 'cc'.
'in' means one lot of letters goes inside another.
'a'+'cc'='acc'
'vine' going around 'acc' is 'VACCINE'.
Can you help me to learn more?
(Other definitions for vaccine that I've seen before include "Immunity provider" , "preparation for pandemic?" , "Immunity stimulant" , "Antibody stimulator" , "Preparation used for inoculation" .)